In the field of structures in mechanical constructions, the objective of producing structures as light as possible, while still ensuring that they are strong and rigid, often results in relatively highly loaded structural parts that include cavities.
The form and composition of such structural parts are very varied, one of the most common forms corresponding to what are called “sandwich” structures having a honeycomb cellular core.
A cellular-core sandwich structure 1, as shown in cross section in FIG. 1, generally comprises a core 2 formed from hollow cells 12, a priori containing air, having on each of its faces, namely the bottom face and the top face, a solid and strong skin panel, respectively 4, 3.
This type of structure, because of its very favorable rigidity-strength/weight ratio, is used particularly in aeronautical constructions.
In one particular embodiment, which is also well known, the skin panels 3, 4 of the sandwich structure are made of composites comprising fibers—glass fiber, aramid fiber, carbon fiber, etc.—held in place in a cured resin, and the hollow cells 12 of the cellular core 2 are formed by means of walls produced in a sheet material shaped so as to define the cells in the form of a usually regular lattice. When this lattice consists of cells of hexagonal cross section, the expression “honeycomb” is generally used.
In most cases, the materials used for the skin panels and for the walls of the cells are relatively impervious to common fluids, resulting in a situation in which each cell constitutes a substantially closed and sealed cavity.
One drawback of this type of structure stems from the fact that the hollow cells are liable to fill up to a greater or lesser extent with water, without this water being able to be removed naturally. This has the effect, on the one hand, of unnecessarily increasing the weight of the structure, intended to be a lightweight structure, and, on the other hand, of reducing, through various physicochemical actions, the strength of the structure by impairing the specific mechanical properties of the materials used in producing the structure or by impairing the quality of the bonding between the various assembled elements, in particular between the cellular material and the skin panels.
The presence of water, or in general of liquid, in hollow cells which is generally considered to be a major defect of the structure, usually cannot be predicted and, whatever the causes of liquid being present—water formation process during manufacture of the structure or subsequent liquid penetration into cells—it is essential to detect the presence of the liquid in cells, to locate where the liquid is and to quantify the amount thereof so as to carry out the necessary actions for removing the largest possible amount of this liquid.
The detection of liquids in such cavitied structures is a long-standing problem. However, the conventional approaches suffer from major drawbacks.
Apart from visual inspections, limited to cases in which the materials used are sufficiently transparent for the liquid in a cell to be observed by optical means, the methods that have been used for the longest time and are well known to those skilled in the art are based on the principle of acoustic detection.
Echographic methods based on this principle are particularly well suited for continuous compact media. In the case of cellular—low-density cavitied—structures, the transmission of an acoustic wave is difficult and is unable to discriminate with the necessary precision the presence of liquid.
A purely acoustic, derived method, suitable for detecting the level of a liquid in a closed space—in this case a bottle—is disclosed in the patent published under the number EP 0 938 653. According to that patent, vibrations are induced in the bottle by means of a magnetic field and an acoustic signal in response to the vibrations is received by a microphone.
The presence and level of liquid in the bottle can be deduced from the characteristics of this received acoustic signal. However, in the case of highly cavitied materials, such as those considered here, this approach does not allow the presence of water in the cavities to be effectively detected because of the weakness of the echo.
Other methods aimed particularly at detecting the presence of water in cellular-core sandwich structures using other physical principles have also been imagined.
According to one of these other methods, since water has thermodynamic characteristics that are very different from the materials in which the presence of water is sought, it has been proposed to produce a thermographic image of a part subjected to temperature variation. In this case, the zones corresponding to water being present vary in temperature less rapidly than the zones where there is no water, because of the differences in thermal inertia between the water and the materials of the part, a thermographic image being capable of detecting the differences in surface temperatures and therefore zones containing water.
Such methods, one example of which is disclosed in the patent published under the number JP 62024134, have however the drawback, on the one hand, of usually requiring the part to be removed, in order for it to be investigated, and, on the other hand, of requiring substantial means for cooling or heating the part under conditions suitable for the measurements to be carried out.
According to another method, the presence of water in the cells is detected by means of electromagnetic microwaves, the propagation of which is modified in the presence of water.
However, the use of electromagnetic microwaves is constricting in terms of the precautions to be taken to protect personnel when the energy level used is significant.
Furthermore, difficulties arise in the case of sandwich structures using skin panels made of a carbon-fiber-based material or having a metallized surface, for example one metallized by means of a bronze mesh, which is the situation for many parts.
In the case of a structure based on carbon fibers, the carbon of the skin panels induces high losses and the signal-to-noise ratio is extremely unfavorable for detection.
When a bronze metallization mesh covers a surface, which is frequently the case for modern aeronautical materials, the skin panels are electrically conducting and prevent any measurements of the dielectric type.
To solve this problem, the solution described in the patent published under the number FR 2 880 424 describes a detection system using electromagnetic microwaves that includes antennas placed in the cellular material of a sandwich panel between the two skin panels. This solution, which is applicable in the case of carbon-fiber-based skin panels, does however require the parts to be modified in order to install the antennas. As a result, the parts are more complex, weaker and heavier, and the solution cannot be easily used for many existing parts or on parts that would not have been provided with these antennas during their manufacture, for cost reasons or for other reasons.
In general, the presence of a bronze mesh prevents any approach of the type in which the level of fluid in the cavities is measured by a radar device (in the radio, microwave or millimeter frequency ranges), since the bronze mesh reflects most of a radio wave in these frequency ranges. To alleviate this drawback, it is usually necessary to reduce the working frequency, but this has the effect of dramatically degrading the precision of the measurements and prevents a measurement of the liquid level from being carried out with sufficient precision.